A “planet” is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

Since a planet must orbit “around the Sun,” excluding bodies around other stars, and a planet must have “cleared the neighbourhood around its orbit” (just try to pin down what that means), the implications of this definition are clear. According to this definition, there are only two planets in the entire universe: Mercury and Venus.

The kids in school today are going to have an easy time memorizing that list.